(a) Digital Micromirror Device
Digital micromirror device (DMD) was developed by Texas Instruments that utilizes the micro electromechanical technology and semiconductor processes to integrate the micro-mechanical structures with CMOS circuits, in order to effectively control the actuating of a single micromirror by addressing by the transistors. The DMD is actuated by the static electricity. When the voltages actuate the DMD, the left and the right side of the mirrors will incline about 10 degrees, respectively. This property could be applied to projector displays and many other components relative to optical valves. The DMD is the representative device of the micro electromechanical system and is already a successful product for business now. From 1987 to this day, the product has been well developed to sixth generation design. From the first to the sixth generation, the structure of the mirrors is developed from the single-layer structure to the double-layer super structure. It not only increases the fill factor of the mirror arrangement, but also enlarges the area of the driving electrode. In the sixth generation, the spring structure even reduces the viscocity and the vibration of the mirrors and substantially improves the performance of the DMD.
The original purpose of the DMD is to be applied to projector displays. The digital light processing (DLP) is the first DMD-type projector display designed by Texas Instruments. The DMD in the DLP projects the light filtered by mirror reflection on the screen. Therefore, every pixel on the screen is composed of one DMD. That is to say, if a projector display has a resolution of XGA (1024×768), the amounts of the DMDs are approximately 800 thousand. The higher the resolution is, the more the DMDs are. Hence, improving the yield rate and reducing the costs are concerned by the industrial field.
Moreover, owing to the keen competition in the display market, the market domination rate of the DLP is hard to break through under the market dominations of the LCD and the plasma display. Therefore, some companies in the industrial field propose to apply the DMD to the optical communication device. Because of the properties of the small size of the mirror which is 16 μm ×16 μm, the space between the adjacent mirrors which is about only 1 μm, and the period of the valving which is 20 μs, the DMD is capable of applying to the optical communication device. If actuating some specific rows of the mirrors, the mirrors will look like blazed gratings from the side view. As shown in FIG. 1, which is a diagram showing the action of the DMD, the Fraunhofer diffraction of the incident light 12 is generated on the DMD 11 by actuating the DMD 11. Moreover, the actuating movement and angle could be controlled by the valves, and thus the DMD has the beam-splitted function of diffracting different wavelengths with different angles. Therefore, the DMD could be served as an optical add/drop multiplexer (OADM) and consequently the value thereof have been raised.
(b) Garting Light Valve
The concept of the grating light valve (GLV) is proposed by Stanford University and made by Silicon Light Machines™. The grating light valve is a grating controlled by valve which is designed according to the property of the light diffraction. As shown in FIG. 2, which is the side view showing the structure of the grating light valve, the grating light valve includes the ribbon structure 21, the common electrode 22, and the air gap 23. The grating light valve consisted of six ribbons 21. The width of each ribbon is about 3 μm, the length of each ribbon is about 100 μm, the thickness of each ribbon is about 125 nm, and the gap between the ribbon and the substrate (i.e. the air gap 23) is only about 650 nm. While the ribbon structure 21 is not actuated, the light travels by the reflection. After the common electrode 22 is actuated by electrifying, the ribbon structure 21 will bend to the air gap 23. As shown in FIG. 2, the grating looks like a square-well grating from the side view. In this way, the different wavelengths could be diffracted with different angles and then applied to the color splits of the display. With respect to controlling the first-order light, when the incident white light travels to the grating light valve, the ribbons will be drawn about λ/4 toward to the air gap 23 so as to adjust the diffraction of the first-order light. It is possible to decide the first-order diffraction angles of different wavelengths by controlling the width of the ribbon structure 21. Therefore, the principle of the actuation of the grating light valve can be applied to the display by this way, as shown in FIG. 3, which is a diagram showing the grating light valves applied to the display. When the incident light 31 is a white light, the first-order diffraction light, which respectively controls the red light 32, green light 33 and blue light 34, mixes the color with the same angle on the display. That is to say, the red pixel 35, the green pixel 36 and the blue pixel 37 can be gratings with different widths and depths according to the wavelengths in order to control the reflective light 38 consisted of the red light 32, the green light 33 and the blue light 34 reflecting to the screen 39 to form the angles and intensity of the different wavelengths. Owing to the displacement of the grating light valve is about λ/4, the displacement needs only 20 ns to carry out. With such a fast displacement, a single row of grating light valve made by scanning could be applied to the display. For example, XGA (1024×768) resolution needs only 768 sets of grating light valves to provide approximately 800 thousand pixels for 2-D display. In comparison with the DMDs, the costs are less and the design is simpler.
Besides, the grating light valve in controlling images could be divided into the digital mode and the analog mode. The digital mode means that the grating light valve is full opened (displaced about λ/4) and closed extremely fast so that the grey scale could be achieved by the ratio of the time of the opening to the time of the closing. The analog mode means that the depth of the displacement of the grating light valve is controlled. The grey scale could be achieved by modulating the intensity of the light between the displacement of 0 and λ/4 in accordance with that the displacement of λ/4 is defined as full opening. With regard to the efficiency of the light, the efficiency could reach 81% because the design is to collect ±1-order light.                (c) The Theory of the Blazed Grating        
The luminous intensity of the diffraction light is generally concentrated on the zero-order diffraction. However, the luminous intensity could be concentrated on particular principal diffraction angle by different blazed gratings. FIG. 4 is a diagram illustrating the diffraction theory of the blazed grating, where θi is an incident angle, θ is any angle at which diffraction light 41 is diffracted, and θb is a blazed angle. It is possible to utilize the inclined plane of the grating 42 so as to acquire the largest luminous intensity on the angle θm which is the principal diffraction angle of the diffraction light 41 concentrated on the m-order diffraction. With respect to the normal N′ of the inclined plane of the grating 42, the desired luminous intensity is similar to the diffraction light that is concentrated on the zero-order diffraction. In this way, assuming that θ=θm firstly, the angle on which the diffraction light 41 is concentrated would be known while θm is acquired. As known from the geometrical optics, according to the normal N′ of the inclined plane of the grating 42, the following equation could be calculated:θi−θb=θm+θb,  equation (1)Besides, according to the diffraction theory, the following equation could be calculated:mλ=a(sinθi+sinθm),  equation (2)where m denotes the order of the diffraction, λ denotes the wavelength of the incident light 43, and a denotes the width of the single grating 42. The angle which the luminous intensity of the diffraction light is concentrated on the m-order diffraction could be obtained through equations (1) and (2). And the following equation could be calculated:mλ=a[sinθi+sin(2θb−θi)],  equation (3)which is the equation of the blazed grating.
Therefore, when designing the blazed grating, the principal diffraction angle on any desired order can be determined by the blazed angle, the incident angle and the width of the single grating so that the luminous intensity of the diffraction light could be concentrated on the specific principal diffraction angle.
Combining the advantages of the blazed grating, the property of the grating light valve, and the structure of the digital micromirror device, a prior blazed grating with valving function is developed, as shown in FIG. 5, which is a side view showing a micro grating structure with a single torsion beam according to the prior art. The micro grating structure includes the silicon substrate 51, the torsion beam 52 and the suspended grating mirror 53. When applying the voltages, the grating mirror 53 is twisted by the torsion beam 52 to be inclined an angle, and thus the diffraction occurs.
However, the grating mirror 53 of the prior blazed grating is actuated merely by the single torsion beam 52. Owing to the processes and the properties of the materials, it has disadvantages that:    (1) When the static electricity actuates a set of the grating mirrors 53, the synchronization of the torsion of the grating mirrors is not steady, which affects the efficiency of the light diffraction;    (2) When the static electricity vanishes, the efficiency of the light reflection is also bad after the return of a set of the grating mirrors 53 owing to the tiny inaccuracy when actuating.
Hence, it is known that a blazed grating having steady actuating and synchronizing is needed, so as to overcome the drawbacks of low efficiency of the light reflection and diffraction in the prior art.